nemo-relay-plugin

nemo-relay-plugin is the Python authoring SDK for NeMo Relay out-of-process dynamic worker plugins. Use it when plugin code should run in its own Python process and communicate with Relay through the versioned grpc-v1 worker protocol.

Relay 0.8 establishes canonical tool results as the grpc-v1 baseline. Workers and custom bindings built for earlier releases must regenerate protobuf bindings, rebuild, and declare compat.relay beginning at 0.8.0. ToolNext.call() returns ToolExecutionResult, preserving opaque annotation metadata independently of the application result JSON.

Authoring Surface

The following rows describe the plugin authoring surfaces available through this SDK.

The worker process isolates Python dependencies and crashes from Relay while preserving host-owned execution, event, scope, cancellation, and shutdown semantics.

Installation

Add the SDK to the Python worker project's dependencies:

uv add nemo-relay-plugin

If you are not using uv, install it with pip:

pip install nemo-relay-plugin

Declare a module:function entrypoint that starts the worker with serve_plugin. Register the plugin manifest through the CLI; Relay creates a per-plugin virtual environment, installs source.manifest_root, and records that environment for activation:

nemo-relay plugins add ./relay-plugin.toml
nemo-relay plugins enable <plugin_id>

Python workers cannot be loaded directly from plugins.toml. They must be registered through plugins add, which provisions the required managed environment. plugins remove <plugin_id> deletes that environment.

Getting Started

A minimal worker plugin looks like this:

from nemo_relay_plugin import Json, PluginContext, WorkerPlugin, serve_plugin


class PolicyPlugin(WorkerPlugin):
    plugin_id = "acme.policy"

    def register(self, ctx: PluginContext, config: Json) -> None:
        async def tag_tool_request(tool_name: str, args: Json) -> Json:
            await ctx.runtime.emit_mark("acme.policy.tool_request", {"tool_name": tool_name})
            if isinstance(args, dict):
                return {**args, "policy": "checked"}
            return {"value": args, "policy": "checked"}

        ctx.register_tool_request_intercept("tag_tool_request", tag_tool_request)


async def main() -> None:
    await serve_plugin(PolicyPlugin())

Marks can also declare a typed data schema, exported-log severity, and semantic category without changing the stable grpc-v1 protocol identifier:

from nemo_relay_plugin import DataSchema, EventCategory, LogSeverity

await ctx.runtime.emit_mark(
    "acme.policy.decision",
    {"allowed": True},
    data_schema=DataSchema("acme.policy.decision", "1"),
    severity=LogSeverity.INFO,
    category=EventCategory.CUSTOM,
)

The original positional emit_mark(name, data, metadata) form remains supported. Relay validates the schema and severity again at the host boundary. Use emit_metric(name, measurements, metadata) to attach the reserved Relay metric schema; authoritative metric validation remains in Relay after mark sanitization.

Use await ctx.runtime.runtime_diagnostics() in an asynchronous callback to read an immutable, host-level RuntimeDiagnostics snapshot. Each entry has code, message, and count; entries is ordered by code and get(code) looks up one entry. Relay aggregates repeated codes, retains the latest message, saturates counts, and caps the snapshot at 32 entries. Entries do not identify the emitting plugin. An older host returns gRPC UNIMPLEMENTED, which the SDK reports as an unsupported-runtime-diagnostics error.

Set load.entrypoint to your_module:main in relay-plugin.toml. Relay imports that function and awaits the returned coroutine when it starts the worker process.

Request Intercepts

LLM request intercepts return one canonical outcome:

from nemo_relay_plugin import LlmRequestInterceptOutcome, PendingMarkSpec


def intercept(model_name, request, annotated):
    del model_name
    headers = {**request.get("headers", {}), "x-policy": "checked"}
    return LlmRequestInterceptOutcome(
        request={**request, "headers": headers},
        annotated_request=annotated,
        pending_marks=[PendingMarkSpec("acme.policy.checked")],
    )

When annotated is present, it is authoritative for provider-body content: leave raw request["content"] unchanged, edit normalized fields or provider extensions through the annotation, and use request["headers"] for transport headers.

Callback Concurrency

The gRPC AsyncIO server can keep multiple RPCs in flight. Callback execution is cooperative: asynchronous callbacks overlap only when they yield control at an await. Synchronous callbacks and synchronous stream iterators run on the worker event-loop thread. Blocking I/O, time.sleep, or long-running CPU work in those callbacks stalls all worker RPCs. Wrap blocking work in an asynchronous callback and offload it with asyncio.to_thread() or another appropriate executor.

The SDK does not configure maximum_concurrent_rpcs, so gRPC does not enforce an application-level RPC admission limit.

Invocation Cancellation

Relay assigns every unary and streaming callback an invocation ID. The host sends CancelInvocation when its managed caller is cancelled, its worker RPC times out, or it stops consuming a worker-backed stream. The SDK cancels the matching asyncio.Task and reports a structured worker.cancelled result.

Cancellation is idempotent. The first request that matches an active callback returns accepted = true; requests for unknown, completed, or already cancelled IDs return accepted = false. Treat acceptance as confirmation that the SDK found and cancelled the task, not as proof that arbitrary user code has stopped.

Python task cancellation is cooperative. Async callbacks should allow asyncio.CancelledError to propagate and use try/finally for cleanup. Synchronous callbacks run on the event-loop thread and cannot be preempted by task cancellation. A blocking synchronous callback can delay both the cancellation RPC and all other worker RPCs, so offload blocking work and make its own cancellation behavior explicit.

grpc-v1 workers are expected to implement this best-effort cancellation contract. Relay remains compatible with older workers that return accepted = false; in that case it still drops the transport request, but it cannot guarantee worker-side interruption.

Windows ARM64 is not currently supported because grpcio does not publish a usable wheel for that platform. The NeMo Relay workspace skips installation and tests for this SDK on Windows ARM64 rather than creating a package without its required gRPC runtime.